1. Field of the Invention
The present invention relates to low-noise, high-stability, deep ultra-violet (DUV), continuous wave (CW) lasers as well as inspection and metrology systems including such lasers.
2. Related Art
Semiconductor inspection and metrology require very stable, low-noise light sources to detect small defects and/or make very precise measurements of small dimensions. UV light sources are important because, in general, shorter wavelengths give better sensitivity to small defects or dimensions.
Low-noise, high-stability lasers are currently available for wavelengths in the visible and near infra-red (IR). However, there are very few CW lasers available for wavelengths in the DUV. Even when available, such lasers are expensive and noisy, have poor long-term stability, and may require frequent adjustments and/or service. Moreover, such lasers typically have powers less than 250 mW, whereas higher powers are desirable for most industry applications because they enable faster and more precise inspection and measurement.
Known DUV CW lasers typically operate by generating a fourth harmonic of an IR fundamental laser. Two frequency conversion stages are typically used, wherein a first stage generates a second harmonic frequency (also called a second harmonic) from a fundamental frequency and a second stage generates a fourth harmonic frequency (also called the fourth harmonic) using the second harmonic frequency. Each frequency-doubling stage (i.e. the first and second stages) uses a non-linear optical (NLO) crystal.
The frequency-doubling process depends on the square of the electric field strength, which is known by those skilled in the art. Therefore, if the power density inside the NLO crystal is low, then the conversion process is very inefficient. An IR laser of a few Watts or even a few tens of Watts of power, when focused into a NLO crystal, produces very little second harmonic because of its low power density. In contrast, a pulsed laser can provide a peak power density many times higher than its average power density. As a result, a pulsed laser of similar time-averaged power density to that of the IR laser can produce substantial amounts of the second harmonic. For example, in some pulsed lasers, roughly 50% of the input of a pulsed laser can be converted to the second harmonic.
A DUV CW laser can use a resonant cavity (also called a cavity) to increase the power density in its NLO crystal, thereby improving its conversion efficiency. Most of the light that passes through the NLO crystal without being converted to the second harmonic is re-circulated in the cavity to build up the power density. Any second harmonic that is produced is allowed to pass out of the cavity. Eventually, the power density builds up to a level where the power leaving the cavity as the second harmonic plus the losses in the cavity equals the input power and so a steady state is reached. To generate the DUV wavelengths, typically two resonant cavities can be connected in series. The first cavity generates the second harmonic (e.g. a visible wavelength, such as 532 nm) by recirculating the IR fundamental wavelength. The second cavity, which is serially coupled to the first cavity, generates the fourth harmonic (e.g. a DUV wavelength, such as 266 nm) by recirculating the second harmonic. Note that the term “coupled” as used to describe the cavities and/or components of the cavities may or may not include components of the cavities physically touching.
FIG. 1 illustrates an exemplary known laser configuration using two cavities, wherein a first cavity implements a second harmonic generator 102A and a second cavity implements a fourth harmonic generator 102B. The second harmonic generator 102A includes a plurality of mirrors 110, 111, 112, and 113 and a NLO crystal 115 to generate the second harmonic. The fourth harmonic generator 102B includes a plurality of mirrors 130, 131, 132, and 133 and a NLO crystal 135 to generate the fourth harmonic. The second harmonic generator 102A can be actively controlled using an oscillator 104 (generating a signal at frequency f1), a modulator 103, a photodiode 105, and a synchronous detector 106. Similarly, the fourth harmonic generator 102B can be actively controlled using an oscillator 124 (generating a signal at frequency f2), a modulator 123, a photodiode 125, and a synchronous detector 126.
IR light (e.g. at 1064 nm) from a fundamental laser 101 enters the second harmonic generator 102A through mirror 110 and, after reflecting from mirrors 111 and 112, enters NLO crystal 115. A portion of the IR light entering NLO crystal 115 is converted to the second harmonic (e.g. to 532 nm). Mirror 113 is coated with a material that reflects the IR light, but transmits the second harmonic. As a result, the second harmonic light passes through mirror 113 and is directed to the fourth harmonic generator 102B.
Most of the IR light passing through crystal 115 emerges from NLO crystal 115 without being converted and thus is reflected by mirror 113 and directed back to mirror 110. Mirror 110 is coated with a material that is highly reflective to the IR light arriving at the angle of incidence of the ray from mirror 113, but is highly transmissive to the incoming IR light from fundamental laser 101.
To build up a high power density in the second harmonic generator 102A, the IR light that has circulated in the first cavity should arrive at mirror 110 in phase with the incoming light from fundamental laser 101. To this end, a servo control can be used to mechanically move mirror 111 to achieve a predetermined cavity length, thereby providing the desired phase. In the configuration shown in FIG. 1, the servo control for the second harmonic generator 102A includes oscillator 104, modulator 103, photodiode 105, synchronous detector 106, and an actuator control 107. Similarly, the servo control for the fourth harmonic generator 102B includes oscillator 124, modulator 123, photodiode 125, synchronous detector 126, and an actuator control 127. An exemplary actuator control can include a piezo-electric transducer or a voice coil to maintain the predetermined cavity length and thus maximize the power density in the cavity.
As shown in FIG. 1, the input IR light from fundamental laser 101 is modulated by modulator 103 at frequency f1 (provided by oscillator 104) to provide a time-varying signal. Note that the coating on any mirror is imperfect, thereby allowing some leakage. As a result, photodiode 105 receives a small portion of the light circulating in the first cavity (i.e. that light reflected by mirror 113 via mirror 110) to provide a signal to synchronous detector 106. Synchronous detector 106 (which could include a mixer or some other similar component) compares the output of photodiode 105 with the output of oscillator 104 at frequency f1 to generate a control signal for actuator control 107. Specifically, synchronous detector 106 can determine whether the length of the first cavity needs to be adjusted and, if so, whether the length should be increased or decreased and by how much. Exemplary servo controls are described in U.S. Pat. No. 5,367,531, as well as in LIGO Technical Note LIGO-T980045-00-D by Black (1998).
A second modulator 123 modulates the input light to the fourth harmonic generator 102B (provided by mirror 113) at frequency f2 to provide another time-varying signal. Photodiode 125 detects a small portion of the circulating light (from mirror 133 via mirror 130). Synchronous detector 106 compares the output of photodiode 125 with the output of oscillator 124 at frequency f2 to generate a control signal for actuator control 127. Specifically, synchronous detector 126 can determine whether the length of the fourth harmonic generator 102B needs to be adjusted and, if so, whether the length should be increased or decreased. Actuator control 127 physically controls the position of mirror 131 to maintain the appropriate length of the fourth harmonic generator 102B so that the phase of the reflected light from mirror 133 is the same as that provided to mirror 130 (via mirror 113).
Thus, the fourth harmonic generator 102B operates in a substantially similar manner to the second harmonic generator 102A except that the input wavelength of the light entering the fourth harmonic generator 102B is the second harmonic (e.g. 532 nm) and the output wavelength is the fourth harmonic (e.g. 266 nm). Note that the coatings and materials of the second and fourth harmonic generator components are chosen appropriately for their respective wavelengths.
In some prior art devices (not shown), second modulator 123 is omitted, thereby resulting in both servo controls operating at the same modulation frequency. In other prior art devices (also not shown), neither first modulator 103 nor second modulator 123 is present. For example, IR laser 101 generates a modulated output by operating the laser such that two modes are generated, those two modes being chosen to have a wavelength separation and relative amplitudes such that an appropriately modulated output is generated by the “beating” of the two modes (see, for example, U.S. Published Patent Application 2006/0176916 by Zanger et al.). Another resonant-cavity servo-control method known in the art that does not need to modulate the laser is that first described by Hänsch and Couillaud in Optical Communications, 35, 442-444, (1980) which uses polarization to measure the phase change in the resonant cavity.
In yet other prior art devices, one or more harmonic generators may comprise two or three mirrors instead of four. In some embodiments, the two cavities may have a different number of mirrors. In yet other prior art devices, the DUV output wavelength may be separated from the recirculating light by a beam splitter (not shown) placed between NLO crystal 135 and mirror 133 (therefore, mirror 133 can be coated with a material to be reflective only).
Notably, the servo controls shown in FIG. 1 can be effective at correcting slow changes in cavity length due to, for example, temperature changes. They may also be effective at correcting cavity length changes caused by low amplitude, low frequency vibrations. Unfortunately, other factors may degrade the output of a cavity that cannot be corrected simply by adjusting the cavity lengths. These factors can reduce the efficiency of the conversion process and lead to a downward trend of the output laser power over time if uncompensated effects change.
Uncompensated effects can include changes in focal length and astigmatism due to spatially varying changes in the properties of the NLO crystal. Such changes may be reversible when caused by photo-refraction at a location due to the power density of the focused beam at that location, or may be irreversible due to damage done to the material of the NLO crystal.
Unfortunately, DUV CW laser 100 can compensate only for changes in cavity lengths. Thus, DUV CW laser 100 cannot compensate for any changes in focus or astigmatism of the NLO crystals in its first or second cavities. Because strongly focused laser light in each NLO crystal typically induces both reversible and irreversible changes in that NLO crystal, DUV CW laser 100 generally operates below optimal intensity and with a shorter lifetime.
Therefore, a need arises for a DUV CW laser that can compensate for any changes in focus or astigmatism of the NLO crystals in its constituent harmonic generators.